It is well known to obtain a suede- or nubuck-like napped leather-like sheet by raising surface fibers of a sheet, in which a fiber substrate is impregnated with a polyurethane resin, using a sandpaper or the like. Characteristics of the sheet to be made can be arbitrarily and widely designed by combination of the substrate comprising fibers and the polyurethane resin. For example, in patent reference 1, it is disclosed that an artificial leather-like product having a very soft touch and is not inferior to a high quality wool fabric which is used for a suit, etc., by using a polyurethane resin comprising polytetramethylene glycol, an organic diisocyanate and a glycol chain extender.
On the other hand, the napped leathery sheet has an appearance and surface similar to those of natural leather, and advantageous features such as uniformity or color fastness, which natural leathers do not have, are recognized, and its applications are spreading in not only for clothing, but also, recently, for long term applications of 5 to 10 years such as covers for furniture such as sofa and covers for car seat.
Therefore, there was a problem that a polyether-based polyurethane resin such as those described in the patent reference 1 cannot be, because it may easily deteriorate by ultra-violet light or by heat, durable for a long term use due to falling off or pilling of surface fibers during use. In addition, although a polyester-based polyurethane resin, which is a polyurethane widely used for leather-like sheet, is good in light resistance to ultra-violet light, etc., its ester bond decomposes by hydrolysis, and there is also a problem to cause and pilling during a long term use.
In the patent reference 2, it is disclosed that a polycarbonate-based polyurethane resin, obtained by reacting a polycarbonate diol, an alicyclic polyisocyanate and an aromatic polyisocyanate, is a useful polyurethane resin for use in which a high durability is required such as for furniture and car seat. However, when the polycarbonate-based polyurethane resin, in which polyhexamethylene carbonate is used, described there, is used as a resin to impregnate into an ultra-fine fiber entangled non-woven fabric, the hand of the sheet becomes plastic-like and stiff. In particular, when the fibers on the surface are raised by a sandpaper or the like, since the polyurethane is too stiff, the surface becomes coarse with short surface fibers, and it is quite difficult to attain a high quality with elegant napped fibers.
In the patent reference 3, for the purpose of making napped fiber quality compatible with durability such as light resistance and hydrolysis resistance, napped leather-like sheets in which a polycarbonate/polyether-based polyurethane resin or a polycarbonate/polyester-based polyurethane resin in which polycarbonate diol, mainly 30 to 90 wt % of polyhexamethylene carbonate diol, and polyether or polyester diol, are used, is disclosed. However, when the ratio of polycarbonate diol is less than 70 wt %, the durability of the product becomes insufficient due to deterioration of polyether component or polyester component, on the other hand, when the ratio of the polycarbonate diol is 70 wt % or more, polyurethane becomes too stiff, and when its surface is buffed with a sandpaper or the like, surface napped fibers become short and coarse, and it was impossible to obtain a napped fiber type leather-like sheet which is satisfactory in napped fiber quality and durability.
On the other hand, in a leathery sheet in which a resin layer is coated or laminated on a fiber substrate, so-called synthetic leather, it is known to use a polycarbonate-based polyurethane resin as the resin layer to improve durability. Mainly for purpose of improving hand of the synthetic leather, various proposals are made for the polycarbonate-based polyurethane to be used for the resin layer.
For example, in patent reference 4, a polyurethane resin in which a copolycarbonate diol made from 1,6-hexane diol and 1,5-pentane diol, is used and a polyurethane resin in which a copolycarbonate diol made from 1,6-hexane diol and 1,4-butane diol is used, are disclosed. In patent reference 5, a polyurethane resin in which a polycarbonate diol made from 2-methyl-1,8-octane diol, is disclosed. In patent reference 6, a polyester/polycarbonate-based polyurethane resin in which both of a polyester diol made from an alkane diol with 5 to 6 carbons and a dicarboxylic acid and a polycarbonate diol made from an alkane diol with 8 to 10 carbons, is disclosed. And, in patent reference 7, a polycarbonate/polyether-based polyurethane resin in which both of a polycarbonate diol made from an alkane diol with 8 to 10 carbons and a polyether diol are used, is disclosed.
When these polyurethane resins are applied to a sheet as an impregnating resin to an ultra-fine fiber entangled non-woven fabric to be present inside spaces of the non-woven fabric, in particular, to a napped leather-like sheet of which surface is buffed with a sandpaper or the like to raise the surface fiber for example, when the copolycarbonate-based polyurethane resin with 4 to 6 carbons described in the patent reference 4 is used, a softening effect to a degree capable of being buffed with a sandpaper cannot be attained, and results in a coarse surface with short surface naps and it is quite difficult to obtain a good quality with excellent naps. Moreover, since the long chain alkylene diol is used, the polycarbonate-based polyurethane resin made from 2-methyl-1,8-octane diol described in the patent reference 5 coagulates too fast when wet-coagulated after impregnation into a non-woven fabric, and voids in polyurethane inside the non-woven fabric become large and coarse, moreover, it causes a partial insufficient foaming, and as a result, when the surface is buffed by a sandpaper, there is a problem that only a product of very poor napped fiber quality with uneven surface nap length can be obtained. In addition, when the surface of the obtained sheet is rubbed with a brush or the like, falling off of fibers was significant, and there was a problem in durability. The polyurethane resins described in patent references 6 and 7, because it uses both of polyester diol and polyether diol, in view of hydrolysis resistance or light resistance, cannot improve durability for a long term use, in particular, the problem of generating pilling, of surface fiber of napped leather-like sheet.
In addition, the technologies described in the patent references 4 to 7 aim to improve surface physical properties such as greasy touch, surface smoothness, tackiness or crack of the polyurethane resin layer provided mainly on the substrate, and they did not consider anything about improving capability of being buffed with a sandpaper or the like when the polyurethane resin is present inside the non-woven fabric, to thereby exhibit preferable ultra-fine fiber nap length, an elegant appearance or flexible surface touch based on the naps, and a soft hand.
On the other hand, in patent reference 8, a soft fibrous sheet in which a thermoplastic polyurethane having a silicone segment in its main chain and/or side chain is used, and a polyurethane-based multi-component fiber suitable for producing the fibrous sheet, are disclosed. When a sheet made thereof is buffed with a sandpaper or the like, since the polyurethane is contained inside the ultra-fine fiber bundle and the polyurethane adheres with the respective single fibers, fiber breakage at buffing is serious and not only insufficient for obtaining an elegant appearance of the napped fibers, but also it was unsatisfactory in softness and hand of sheet.
Furthermore, in patent reference 9, a sheet and a production method thereof in which a polyurethane resin modified with a silicone polyol having, based on polymer diol component, 5 to 30 wt % dimethyl siloxane structural unit, is present in a state such that it is substantially not present inside the ultra-fine fiber bundle which constitutes the non-woven fabric. However, as for the silicone polyol, there is only a description about a dimethylsiloxane structure with hydroxyl group at both ends. This polyurethane resin obtained by using the silicone polyol having hydroxyl group at both ends is a block copolymer in which the dimethyl siloxane chain is introduced to the polyurethane main chain. When a sheet impregnated with the polyurethane resin comprising, as main component, the polycarbonate modified with the silicone polyol having-hydroxyl group at both ends, is subjected to a raising treatment by an ordinary buffing, in order to obtain a preferable nap fiber length and an elegant appearance, it becomes necessary that the content of the silicone polyol is at least 10 wt % or more based on the polymer diol comprising polycarbonate diol as main component, but when 10 wt % or more silicone polyol is contained in the polyurethane resin, the light resistance of the polyurethane resin worsens greatly to thereby generate fluff or pilling of fibers and a product satisfactory in durability was not obtained.
As stated above, by conventional techniques, it is quite difficult to stably produce a napped leather-like sheet excellent in every of hand, nap fiber quality and durability.
[Patent reference 1] JP-A-S59-192779
[Patent reference 2] JP-A-H3-244619
[Patent reference 3] JP-A-2002-30579
[Patent reference 4] JP-A-H5-5280
[Patent reference 5] JP-A-H2-33384
[Patent reference 6] JP-A-H4-300368
[Patent reference 7] JP-A-H5-9875
[Patent reference 8] JP-A-H4-202861
[Patent reference 9] JP-A-H7-150478